A transponder system is described, in which with the aid of an interrogation device batteryless transponders may be caused to transmit an answer signal, which may contain encoded information as regards the identity of the respectively reacting transponder and possibly further information as well. The particular feature of the transponder utilized is that it does not comprise any batteries to supply its operating power. The operating voltage which the transponder requires in order to transmit its response signal is obtained from a RF pulse interrogation which is transmitted by the interrogation device of the transponder system. In the transponder this RF interrogation pulse is rectified and the voltage then produced is utilized for charging a capacitor constituting an energy storing means. As soon as circuit unit in the transponder detects the end of this RF interrogation pulse and sufficient energy has been stored in the energy storing element, the transponder transmits the above noted answer signal. This answer signal may then be received and processed by the interrogation device.
Such transponders of the type described may, for instance be implanted in animals or by arranged on articles so that with the aid of the interrogation device, the animals or the articles may be identified on the basis of the encoded information in the answer signals.
One problem in conjunction with such a transponder system occurs if a plurality of transponders are present in the transmission range of the interrogation device. Such transponders then namely simultaneously receive one transmitted RF interrogation pulse and will then also transmit their answer signal back at the same time as well, if after the end of the RF interrogation pulse there is sufficient energy stored in their energy storing means. The simultaneously produced answer signals render unambiguous identification of the respective transponder by the interrogation device impossible.
In addition, if instead of the interrogator transmitting one high power interrogation pulse, which would provide at least the furthest transponder with enough energy to respond, the interrogator transmitted a successive series of low power to high power pulses, wherein the low power pulse would be enough energy to charge-up the closest transponder and the high power pulse would be enough energy to charge-up the furthest transponder, transponders will still simultaneously respond. To illustrate an example, assume that the transponder has a discharge function such that if the transponder does not receive adequate power to transmit an entire response telegram upon the termination of the interrogation pulse, the transponder discharges. Then, when the interrogator transmits a minimum power pulse, only the closest transponder is adequately charged with enough energy to respond, so there is no interference from other transponders trying to respond simultaneously. However, when the interrogator transmits a higher power interrogation pulse, not only are the further transponders charged-up adequately to respond, but the closer transponders are also charged up adequately to respond, thereby causing interference in the reception of either answer signal.